System for power control in cells for electrolytic recovery of a metal

ABSTRACT

Disclosed is a system for electrolytic processing or recovery of a metal from an electrolyte solution. The system may comprise electrolysis cells and a rectifier. The cells may comprise interleaved anodes and cathodes. The anodes or the cathodes of a first cell may have an electrical connection to a terminal of the rectifier, respectively, via a first electrical path having a first resistance. The anodes or the cathodes of a second cell may have an electrical connection to a terminal of the rectifier, respectively, via a second electrical path having a second resistance. The second resistance is configured to be higher than the first resistance. The system may further comprise a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell.

CROSS-REFERENCE TO RELATED APPLICATION

This is a national stage application filed under 35 USC 371 based on International Application No. PCT/FI2011/050385 filed Apr. 28, 2011, and claims priority under 35 USC 119 of Finnish Patent Application No. 20100184 filed Apr. 30, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the electrolytic processing of metals. Examples of electrolytic recovery of metals are electrorefining and electrowinning. The invention relates particularly to control and distribution of electrical power within a single electrolytic cell or distribution of electrical power between multiple electrolytic cells.

2. Background of the Invention

In electrolytic recovery of metals an electric current is passed between two electrodes immersed in an electrolyte which is a solution containing the metal in a dissolved ionic form. The electrical current causes the metal to be deposited on the cathode.

Electrorefining (ER) is an electrolytic process for purifying a metal. An impure metal anode is dissolved and the pure metal is deposited at the cathode. The anode is made electrically positive and the cathode is made negative by application of an external voltage, so that an electrical current passes through the electrolyte between the anodes and cathodes. For example, in the electrorefining of copper, the anode is made of impure copper, the copper enters the electrolyte as the anode dissolves anodically giving copper (II) ions (Cu2+(aq)) historically referred to as “cupric” ions. Typically, the electrolyte contains copper as copper sulfate with sulfuric acid as a supporting electrolyte. The copper (II) ions are transported through the electrolyte and reduced at the cathode, where pure copper is deposited. Impurity elements from the anode can remain as solids and deposit in the anode slimes in the cell, or they can dissolve in the electrolyte. Impurity elements comprise, for example, nickel or arsenic.

In copper electrorefining plants, the concentration of impurity elements such as nickel and arsenic typically build up with time, and liberator cells are typically used for purifying the tankhouse electrolyte from the main production process. Electrowinning (EW) is an electrolytic process for recovering dissolved metals from an electrolyte. A number of metals can be won from solution using electrolytic methods. These metals include but are not limited to copper, nickel, gold, silver, cobalt, zinc, chromium and manganese.

In industrial electrowinning of copper, the cell voltage is usually approximately 2.0V, the current density can be in the range 200 to 400 Amperes per square meter and the area of each electrode face is usually about 1 square meter. The term cell is used to describe a tank in which at least one anode and one cathode are immersed in an electrolyte solution which is usually aqueous. In the following description the term tankhouse means an arrangement, wherein at least one cell (or tank) and power source are present in a building or enclosed structure, that is, a house. In a typical configuration a tankhouse comprises a plurality of cells.

When using a plurality of cells in conventional electrowinning, it is usual to have several cells powered by the same rectifier, giving the same current density at each cathode.

Liberator cells are similar to standard electrowinning cells. During the process copper is plated out on the cathodes and copper ion concentration in the electrolyte decreases.

In certain technical solutions of the electrolytic recovery of metals, the state of the electrolyte changes during the process. For example, in a purification process of the electrolyte from copper electrorefining, it is desirable to remove most of the copper ions along with harmful impurities from the electrolyte. The properties of the electrolyte—especially the copper concentration—change with time during the process.

In later stages of electrolytic recovery of metals in liberator cells, where copper concentration is low, if the current density is too high, the result can be the undesired production of a powdery copper deposit (or copper sludge) at the cathode. There is also a risk of toxic arsine gas production at the cathode surface.

In terms of electrode materials, liberator EW cells are similar to standard electrowinning cells, the anodes are insoluble. For copper liberators the anodes are usually lead-based alloys; rolled lead-calcium-tin alloy, or antimonial lead. Mixed metal oxide (MMO) coated titanium anodes—also known by the trademark name of Dimensionally Stable Anode™ (DSA) may also be used.

The cathodes in liberator cells are usually spent anodes from the electrorefining tankhouse, but can also be permanent cathodes with stainless steel blades. Older refineries may still use copper starter sheet technology.

Copper cathodes deposited in the liberator cells which contain impurities (such as arsenic and antimony) are returned to the smelter to be melted and cast into anodes for electrorefining.

Decopperised electrolyte can be sent to the electrorefining cells, or further processed, for example, for nickel removal.

In certain technical solutions of electrolytic recovery of metals, the state of the electrolyte inevitably changes during the process. For example, in a purification process of the electrolyte it is desired to remove all copper together with harmful impurities from the electrolyte. This means that properties of the electrolyte change with time during the process.

Liberator cells are similar to normal electrolytic cells, but they have lead anodes in place of copper anodes. Copper in the solution is deposited on copper starting sheets. As the copper in the solution is depleted, the quality of the copper deposit is degraded. Liberator cathodes containing impurities such as antimony are returned to the smelter to be melted and cast into anodes. Purified electrolyte is recycled to the electrolytic cells.

In a liberator tankhouse, the aim is to remove copper from the solution as a solid copper cathode deposit. The current densities employed are typically lower than in standard copper electrowinning or Electrorefining. As the process proceeds, the copper concentration of the electrolyte gets down to lower cation concentrations.

Reference is now made to FIG. 1 which illustrates a system for electrolytic recovery of metals in prior art. In FIG. 1 there is a voltage source 100 which provides a negative voltage to conductor 102 which is further connected to a cathode busbar 118. Voltage source 100 provides a positive voltage to conductor 104 which is further connected to anode busbar 129 of a cell 120. There are two electrolytic cells, namely a cell 110 and cell 120. Cell 110 comprises a cathode 114 and an anode 116. Cell 110 contains an electrolyte solution. Cell 120 comprises a cathode 124 and an anode 126. Cell 120 contains an electrolyte solution. Cathode busbar 118 is connected to cathodes in cell 110 such as cathode 114. Cathode busbar 128 is connected to cathodes in cell 120 such as cathode 124. Anode busbar 119 is connected to anodes in cell 110 such as anode 116. Anode busbar 129 is connected to anodes in cell 120 such as anode 126. Cell 110 and cell 120 are connected electrically in series so that anode busbar 119 is connected along its length to cathode busbar 128.

During the liberation process the electrolyte has initially high copper concentration. As the electrolyte is processed the copper concentration decreases and acid concentration increases. In prior art solutions, the maximum current density which can be used is dictated by the copper concentration in the lean electrolyte, since all the cells are connected electrically in series and carry approximately the same current.

In prior art a cell contains a plurality of anodes, all connected electrically in parallel and a plurality of cathodes also connected electrically in parallel. The voltage across a cell is therefore approximately equal to the voltage that would be experienced between a single anode and a single cathode. By way of example, this voltage would be approximately 1.7 to 2.8 Volts in the case of the electrowinning of copper, depending on the current density employed.

It is difficult to convert electrical power from mains voltage to a dc voltage of this magnitude efficiently. For this reason it is common practice to connect cells in series so that they all conduct the same current, but the voltage across the series chain of cells is equal to the sum of all the cell voltages. By this means the voltage rating of the central dc current source, commonly called a rectifier, is elevated and high efficiency can be obtained.

The difficulty with this arrangement is that the same current density is used in all cells. Cells may operate at a much lower current density than the copper concentration in the electrolyte would permit. This causes that there are more cells compared to a situation where the process would be run using optimum current densities. Thus, a liberator tank house uses more electrical power than in an optimum case.

SUMMARY OF THE INVENTION

According to an aspect of the invention, the invention is a system for electrolytic processing of a metal comprising at least two electrolysis cells for the metal and a rectifier, wherein the at least two cells comprise at least three anodes and at least two interleaved cathodes. For the system is characteristic that the anodes or the cathodes of a first cell have an electrical connection to a positive or a negative terminal of the rectifier, respectively, via a first electrical path having a first resistance; the anodes or the cathodes of a second cell have an electrical connection to a positive or a negative terminal of the rectifier, respectively, via a second electrical path having a second resistance; the second resistance is configured to be higher than the first resistance; and the system further comprises a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell.

According to another aspect of the invention, the invention is a system for electrolytic processing of a metal comprising at least two electrolysis cells for the metal and a rectifier, wherein a first cell comprises a plurality of cathodes interleaved between a plurality of anodes and a second cell comprises a plurality of cathodes interleaved between a plurality of anodes. For the system is characteristic that the anodes of the first cell have an electrical connection to a positive terminal of the rectifier via a first electrical path having a first resistance; the anodes of the second cell have an electrical connection to a positive terminal of the rectifier via a second electrical path having a second resistance; the number of anodes and cathodes in the second cell is configured to be higher than the number of anodes and cathodes in the first cell to diminish a difference between the first resistance and the second resistance; and the system further comprises a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell.

In one embodiment of the invention, the configuration of anodes and cathodes in the first and second cells is such that a cathode plate is placed between two anode plates. The cathode and anode plates may be substantially parallel in a cell. The plates may be rectangular, for example, 1 meter by 1 meter.

The distances from cathodes to neighboring anodes, between which the cathodes are arranged, may be substantially same. By substantially the same distances may be meant a difference of less than 10 centimeters in the distances. By substantially parallel may be meant at most an angle of 10 degrees between plates.

In one embodiment of the invention, the at least two cells comprise the at least three anodes and the at least two interleaved cathodes, the cathodes being interleaved between the anodes. Thus, between two anodes there is a cathode.

In one embodiment of the invention, the anodes of a first cell have an electrical connection to a positive terminal of the rectifier, respectively, via a first electrical path having a first resistance, the anodes of a second cell have an electrical connection to a positive terminal of the rectifier, respectively, via a second electrical path which has the second resistance.

In one embodiment of the invention, the cathodes of a first cell have an electrical connection to a negative terminal of the rectifier, respectively, via a first electrical path having a first resistance, and the cathodes of a second cell have an electrical connection to a negative terminal of the rectifier, respectively, via a second electrical path which has a second resistance.

In one embodiment of the invention, the first electrical path and the second electrical path comprise metal conductors.

In one embodiment of the invention, the first electrical path consists of conducting material and the second electrical path comprises at least one resistor device in addition to at least one conductor.

In one embodiment of the invention, the first electrical path comprises conducting material and the second electrical path comprises at least one resistor device.

In one embodiment of the invention, the second electrical path comprises a resistor and an anode or a cathode bar in series, the anode or the cathode bar being connected to each anode or cathode, respectively, of the second cell.

In one embodiment of the invention, the second electrical path comprises a resistor and an anode bar in series, the anode bar being connected to each anode of the second cell.

In one embodiment of the invention, the second electrical path comprises a resistor and a cathode bar in series, the cathode bar being connected to each cathode of the second cell. In one embodiment of the invention, the second electrical path comprises an anode bar to which are connected electrical paths for each of the at least three anodes of the second cell, the electrical paths for each of the at least three anodes having respective resistors.

In one embodiment of the invention, the second electrical path comprises an anode or a cathode bar, respectively, of the first cell.

In one embodiment of the invention, the metal is copper.

In one embodiment of the invention, the first cell and the second cell are liberator cells.

In one embodiment of the invention, the system further comprises an intermediate voltage supply configured to supply local converters. The local converters are connected to anode or cathode busbars of a cell. There may be local converters for each cell in the system. The local converters may be connected to a number of cells.

In one embodiment of the invention, the electrolytic process is electrowinning or electrorefining. In one embodiment of the invention, high cathodic current density is used in the first cell in the process, where copper concentration is high and lower cathodic current densities are used in the second cell where copper concentration is lower. In one embodiment of the invention, there is used a separate voltage supply, such as a local converter, on every cathode that would make control of current density in each individual cell, each individual cathode, group of cells or sections of rectifiers much easier.

In one embodiment of the invention an external resistance is used to control the distribution of current between at least two cells connected in parallel, in that way each cell would have a cathode with a different current density.

In one embodiment of the invention, there is a power management system for a tank house for electrorefining and electrowinning. The system comprises a plurality of cathode and anode pairs arranged into at least one cell and a plurality of voltage supplies coupled to each of the cells. The plurality of voltage supplies are configured to supply voltage to said cells as a response to the properties of the electrolyte at each cell. The properties include, for example, copper concentration and acid concentration and the properties may vary within a tankhouse comprising a plurality of cells.

In one embodiment the voltage supplies are local converters. Voltage supplies mentioned above are configured to supply each of the pairs individually or they may also be configured to supply a group of pairs or a portion of a cell.

In one embodiment of the invention cells are liberator cells. In one embodiment of the invention the power management system further comprises an intermediate voltage supply configured to supply local converters.

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. A system or an apparatus to which the invention is related may comprise at least one of the embodiments of the invention described hereinbefore.

It is to be understood that any of the above embodiments or modifications can be applied singly or in combination to the respective aspects to which they refer, unless they are explicitly stated as excluding alternatives.

The benefit of the present invention is that it is possible to use the best possible current density in electrowinning and electrorefining when done in a liberator cell or similar cell in which it is not beneficial to maintain the same current density at each of the cells. For example, when the concentration of copper is low, high current densities cannot be used because of the risk of producing a copper powder deposit or arsine gas. The present invention achieves different current densities in different cells in the same tankhouse. A further benefit of the present invention is that it provides better control of electrowinning and electrorefining processes when the current density can be chosen such that it will provide best possible results at the given conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

FIG. 1 is a block diagram of a system for electrolytic recovery of metals in prior art;

FIG. 2 is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anodes having individual resistors, in one embodiment of the invention;

FIG. 3A illustrates a resistor in one embodiment of the invention;

FIG. 3B illustrates a parallel combination of a resistor and a transistor in one embodiment of the invention;

FIG. 3C illustrates a transistor in one embodiment of the invention;

FIG. 4 is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anode busbar having a shared resistor, in one embodiment of the invention;

FIG. 5 is a block diagram illustrating an alternative system for resistive control in a liberator arrangement, in one embodiment of the invention;

FIG. 6 is a block diagram showing a system employing a central rectifier to produce two current paths, in one embodiment of the invention;

FIG. 7 illustrates mounting of linear regulators on anode hanger bars, in one embodiment of the invention;

FIG. 8 shows a method of connecting cell sections such that several current densities are provided, in one embodiment of the invention;

FIG. 9 is a block diagram showing a system where different current densities are provided for upstream and downstream cells with respect to the electrolyte flow using serial connection of cells, in one embodiment of the invention; and

FIG. 10 is a block diagram showing an alternative method of connecting the cells or cell sections in which the polarity of the current bars on either side of the cells are swapped over, in one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

FIG. 2 is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anodes having individual resistors, in one embodiment of the invention.

In FIG. 2 there is a power supply 240, the negative terminal of which is connected to conductor 202 which is further connected to a cathode busbar 218 of a cell 210 and a cathode busbar 228 of cell 220. Thus, cathode busbars 218 and 228 are connected in parallel. The positive terminal of power supply 240 is connected to conductor 204, which is further connected to an anode busbar 219 of cell 210 and an anode busbar 229 of cell 220. Thus, anode busbars 219 and 229 are connected in parallel. A potential difference or voltage is applied between conductor 202 and conductor 204, such that there is a potential difference between resistor 217 and cathode busbar 218. The same potential difference is applied, in parallel, between resistor 227 and cathode busbar 228. In this way electrodes connected to resistor 217 and resistor 227 are held at an anodic potential. Cathode busbar 218 and cathode busbar 228 are held at a cathodic potential. Cell 210 comprises a number of cathodes such as a cathode 214 and a number of anodes such as an anode 216. The number of anodes in cell 210 is one larger than the number of cathodes. The anodes and cathodes may be plates. Cell 210 contains electrolyte 212, when the cell is used for electrolysis. Cell 220 comprises a number of cathodes such as a cathode 224 and a number of anodes such as an anode 226. Cell 220 may contain electrolyte 222. In cell 220 the number of anodes is one larger than the number of cathodes. The anodes and cathodes may be plates, for example, 1 meter by 1 meter plates, in cells 210 and 220. Cathode busbar 218 is connected to cathodes in cell 210 such as cathode 214. Cathode busbar 228 is connected to cathodes in cell 220 such as cathode 224. Anode busbar 219 is connected to anodes in cell 210 such as anode 216 via resistors such as resistor 215 and resistor 217. The resistance in resistors between anode busbar 219 and anodes in cell 210 may be low or the resistors are optional. Anode busbar 229 is connected to anodes in cell 220 such as anode 226 via resistors such as resistor 225 and resistor 227. The resistances in resistors 215 and 217, which are connected to the anodes at the ends of cell 210, have a higher resistance compared to the resistance in other resistors between anode busbar 219 and the respective anodes. Similarly, resistances in resistors 225 and 227, which are connected to the anodes at the ends of cell 220, have a higher resistance compared to the resistance in other resistors between anode busbar 229 and the respective anodes. This is due to the fact that the current to anodes at the ends of cells 210 and 220 is lower since these anodes face only one cathode, on one side. The difference in the resistance between a resistor connected to an anode at a cell end and a resistor connected to an anode located between two cathodes is proportional to the difference in current caused by an anode plate facing only a single cathode plate instead of two cathode plates. A pipe 230 provides electrolyte to cell 210. Pipe 232 provides the electrolyte to from cell 210 to cell 220. The processed solution exits from pipe 234. The arrows 231 and 233 indicate the direction of the electrolyte flow. As may be seen from FIG. 2, the solution flows in series through the cells. Power supply 240 may be a step down Direct Current (DC) to DC converter. Power supply 240 may comprise a direct current source 242, an inductor 244, a capacitor 246, transistor Q1 and Q2. Transistors Q1 and Q2 are acting synchronously in anti phase. Power supply 240 may incorporate a switching regulator in order to be highly efficient in the conversion of electrical power. The switching regulator may be a buck circuit. Power supply 240 may also be a central rectifier.

Electrically, cell 210 and 220 are not connected in series but in parallel. In general, all anode busbars are connected to the positive terminal of the central rectifier, that is, power supply 240. All the cathode busbars, namely, busbar 218 and 228 are connected to the negative terminal of central rectifier 240. Cathodes of cell 220 operate at a lower current than those in the first cell 210 as a result of having a resistance connected in series with each cathode or anode. The current in the cathodes decreases progressively in the cell chain, for example, from 600 Amps in the first tank to 200 Amps in the last tank.

In one embodiment of the invention, cell 210 and cell 220 are liberator cells. Cell 210 represents a first stage of a liberator cell circuit and cell 220 represents a second stage of a liberator cell circuit.

In the first stage the electrolyte is distributed in cascade through at least cell 210. There may also be at least one other cell in the first stage. In the second stage the electrolyte is distributed in cascade through at least cell 220. There may also be at least one other cell in the second stage. By the use of two stages may be achieved a decrease in the copper concentration from an initial value of 40-60 g/dm3 in the feed solution down to 10-15 g/dm3. In the second stage copper is removed from 10-15 g/dm3 down to ca. 1 g/dm3. In the first stage copper may be removed from the solution as solid copper, which is deposited on the cathodes. The electrolyte is cascaded through the liberator (EW) cells, and an electrical current is applied. The current densities employed are set to be lower than in standard copper electrowinning or electrorefining.

Thus, when the electrolyte is cascaded through a plurality of anode-cathode pairs the current density is preferably controlled in order to get the best possible result.

As metallic copper is deposited on the cathode surface, the copper concentration in the electrolyte solution is depleted and the quality of the copper deposited at the cathode can decrease.

In one embodiment of the invention, the resistors may be in series with the cathodes instead of the anodes. For example, so that each of cathodes in cell 220 are connected to cathode busbar 228 via their own resistors.

The different currents are obtained by using different values of series resistor. For better current control, the resistor can be replaced by a resistor and transistor, typically a power MOSFET, in parallel with the resistor operating as a controlled resistor and providing fine control of the current. Alternatively the resistor can be replaced entirely by a transistor to give complete current control. These options are illustrated in FIG. 3. FIG. 3A shows a resistor alone. FIG. 3B shows a parallel combination of resistor and transistor (power MOSFET, Metal-Oxide-Semiconductor Field-Effect Transistor). FIG. 3C shows a transistor (power MOSFET) alone.

In the embodiment of the invention, an external resistance is used to control the distribution of current between two or more cells connected in parallel. In that way each cell would have a cathode with a different current density. The concept is to use external resistances to divide the current from a single rectifier, such that different current densities can be obtained in different cells (or cell sections) in the process. The resistors in FIG. 2 are just an example of means for providing desired current to each of the cells. A person skilled in the art understands that this may be provided also by using different means, such as local convertors. The external resistances would be of differing values and electrically connected before the anodes in the process in order to control the distribution of current between cells connected in parallel. The external resistance may also be adjustable. In that way each cell (or section of cells) would have cathodes with a current density which is a function of the external resistance.

In a copper liberator cell house, wherein it is desired to remove as much copper as possible from the electrolyte solution, it should be possible to divide current from a single power supply such that a high current density (e.g. 300 A/m2) can be applied in the first cells where Cu concentration is high and a lower current density in the last cells where Cu concentration is low (e.g. 100 A/m2). In intermediate cells 200 A/m2 might be used. In this way it will be possible to gain good current efficiency in each set of cells, so that use of electrical power is optimized.

FIG. 4 is a block diagram of a system for electrolytic recovery of metals where anode and cathode busbars for two cells are connected in parallel, the anode busbar having a shared resistor, in one embodiment of the invention.

In FIG. 4 there is a power supply 240 as disclosed in FIG. 2. The power supply provides a negative voltage to conductor 402 which is further in parallel connected to a cathode busbar 418 of a cell 410 and a cathode busbar 428 of cell 420. Power supply 440 provides a positive voltage to conductor 404 which is further connected to an anode busbar 419 of cell 410 and an anode busbar 429 of cell 420. Cell 410 comprises a number of cathodes such as a cathode 414 and a number of anodes such as an anode 416. Cell 410 may contain electrolyte 412. Cell 420 comprises a number of cathodes such as a cathode 424 and a number of anodes such as an anode 426. Cell 420 may contain electrolyte 422. In both cells the number of anodes is one larger than the number of cathodes. Cathode busbar 418 is connected to cathodes in cell 410 such as cathode 414. Cathode busbar 428 is connected to cathodes in cell 420 such as cathode 424. Anode busbar 419 is connected to anodes in cell 410 such as anode 416. Anode busbar 429 is connected to anodes in cell 420 such as anode 426. Anode busbar 429 is connected to conductor 404 via a resistor 427. A pipe 430 provides electrolyte to cell 410. Pipe 432 provides the electrolyte to from cell 410 to cell 420. The processed solution exits from pipe 434. As may be seen from FIG. 4, the solution flows in series through the cells.

FIG. 5 illustrates an alternative method for incorporating resistive control in a liberator arrangement, in one embodiment of the invention.

In the arrangements shown in FIGS. 2 and 4, all cathodes are in parallel and all anodes are in parallel. This requires a central rectifier of a low-voltage, high-current output. The voltage is approximately that of a single cell. When the number of cathodes and anodes is large, the magnitude of the rectifier current may be inconveniently large. It is then advantageous to use a series arrangement of cells so that the central rectifier voltage becomes larger and its current rating smaller for a given power output. FIG. 5 illustrates such an arrangement. In FIG. 5 there is a rectifier 540 and cells 510, 511, 512, 513, 514 and 515. The rectifier has a positive terminal 204 and a negative terminal 202. The cells have been formed from three larger tanks, such as a combination of cells 510 and 515 would have been. The cells may also be formed by dividing the tanks into two individual cells using barriers 540, 541 and 542. However, cells 510 and 515 share anode busbar 520 and cathode busbar 521. Similarly, cells 511 and 514 share anode busbar 522 and cathode busbar 523. Similarly, cells 512 and 513 share anode busbar 524 and cathode busbar 525. Cathode busbar 521 and anode busbar 522 is connected using conductor 550, whereas cathode busbar 523 and anode busbar 524 is connected using conductor 551. Thus, the barrier separated cell pairs are connected in series. The cell electrical current in cells 510 and 511 may be higher than in cells 512 and 513. Respectively, the cell electrical current in cells 512 and 513 may be higher than in cells 514 and 515. The flow of electrolyte is illustrated with arrows 530, 531, 532, 533, 534, 535 and 536. Electrical current flows from positive terminal 204 of rectifier 540 to a negative terminal 202 of rectifier 540.

In cells 515, 514 and 513 resistors (not shown) are connected in series with the anodes or cathodes to regulate the flow of current through these cathodes. More accurate control over the current value is obtained by the use of resistors with transistors in parallel or by transistors alone as previously described. The resistor values are chosen so that the total current taken by each cell divides between the upper and lower sections of the cell in the desired ratio for each cell. The resistor values differ for each cell so that there is a gradation of current density experienced by the electrolyte as it flows through the series of cell sections. It will be appreciated that, if required, the cells can be separated electrically and not joined by an equalizer-type arrangement incorporating the cathode and anode bars. In that case a single resistor can be used for the upper and lower sections of the tank in a similar manner to that set out with respect to FIGS. 2 and 4.

In one embodiment of the invention, each tank in FIG. 5 is divided into two equal halves by a barrier such as barriers 540, 541 and 542, which separate the electrolyte in the two halves in the tanks. The flow of electrolyte is illustrated with arrow 502. However, the cathode busbars and anode busbars along the side of the cells are continuous.

In one embodiment of the invention, instead of two cell sections, two separate cells can be used and the cathode bars and anode bars of these can be electrically connected. An equalizer bar type arrangement can be used to join cells in series. There will be more longitudinal current flow along the equalizer bars than is usual in prior art arrangements. Extra anodes will be required at the ends of cell sections. The electrolyte flows through the cells using one half of the cell, for example, the upper halves in FIG. 5, and flows in a contrary direction through the other cell half sections, for example, the lower halves in FIG. 5. Current flows from a rectifier positive terminal 204 to a rectifier negative terminal 202. The cell voltages are additive.

FIG. 6 shows a further embodiment of the invention in which a rectifier 640 is employed to produce two parallel current paths, in one embodiment of the invention. In place of rectifier 640 power source 240 of FIG. 2 may also be used.

In FIG. 6 the first current path goes via cells 610-614 and the second current path via cells 620-624. In neighbouring cells, such as cells 610 and 611, the anode and cathode busbars are connected via an electrical conductor. The cells are divided in two equal sections as illustrated in FIG. 6 by the separation of cells in first cells 610-614 and second cells 620-624. The electrolyte flow is illustrated with arrows 650, 652 and 654. In this case, however, the cathode bars and anode bars are not continuous along the length of two cell sections or two cells, such as cells 610 and 620, but are also divided. The arrangement therefore may be described as a two series of half-length cells. Resistors 632, 634 and 636 are employed to produce an exchange of current between the first and the second current paths. As the concentration of the target metal ion (e.g. copper) in the electrolyte decreases in the upper series of cells 610-614, the current in this path is decreased by diverting part of the current to the lower current path. Similarly, current is added to the lower current path where it meets electrolyte of with higher concentrations of the target metal ions. The cell sections or half cells can be connected by two busbars 660 and 662. Busbar 662 connected the cathodes of cells 614 and 624 to negative terminal 202 of rectifier 640 while busbar 660 connected the anodes of cells 610 and 620 to the positive terminal 204 of rectifier 640. There will be more longitudinal current flow than is usual when equaliser bars are used in prior art arrangements. Resistors, transistors or resistors in parallel with transistors may be used as the current diverters. Resistors 632, 634 and 636 could be a switched-mode converter in which case the losses could be less that when 632, 634 and 636 are resistors.

FIG. 7 shows how transistors acting as current-mode linear regulators may be mounted on the anode or cathode hanger bars as an alternative to mounting current-controlling elements on the sides of the cells, in one embodiment of the invention.

The use of on-board linear regulators is particularly applicable to the cell arrangement shown in FIGS. 2 and 5. Cathodes or anodes in which the current is to be regulated (shown shaded in FIGS. 2 and 5) may be replaced by current regulated electrodes of the design illustrated in FIG. 7 in which current passes between the hanger bar 713 and the electrode blade 714 via transistors 715-719 (typically power MOSFET transistors). These transistors operate in the linear regime to control current flow between the hanger bar and the electrode blade.

FIG. 8 shows a method of connecting cell sections such that several current densities are provided as might be advantageous in a liberator EW process, in one embodiment of the invention. The numbers 1-30 in each cell in columns indicate the cathodes. The accompanying numbers by the side of numbers 1-30 in columns indicate the current in amperes flowing through each cathode. In this illustration, the series connection of cells and cell sections draws from the central rectifier positive terminal 808 a current of 6,000 Amps which is returned via the central rectifier negative terminal 809. The electrical current paths between cells are illustrated with arrows 811. The number of cathodes in each section is adjusted according to the current density to be employed in that section. Extra anodes will be required at the ends of the sections. Cells are divided into sections by dividers 810 as are the anode bars and cathode bars. Electrolyte flow 802 takes the electrolyte around these barriers. Anode bars and cathode bars are connected in sequence by busbars or cables.

FIG. 9 shows a system where cells are separated into three stages, in one embodiment of the invention. In FIG. 9 there is a rectifier 540. Rectifier 540 provides a negative voltage via negative terminal 902 to a cathode bar 918 to which a cathode 914 is connected within a cell 910. Rectifier 540 provides a positive voltage via positive terminal 904 to an anode bar 966 to which anodes are connected within a cell 960. An anode 912 of cell 910 is connected to an anode bar 916. Anode bar 916 is connected to a cathode bar 928 of cell 920. Anodes in cell 920 are connected to anode bar 926. Electrolyte flows from cell 910 to cell 960 via cell 920, cell 930, 940 and cell 950. The cells are electrically connected in series. A series connection of cells draws from a positive terminal 904 of rectifier 540 a current of, for example, 6000 Amps which is returned via the negative terminal 902 of rectifier 540. The number of cathodes in each cell is adjusted according to the current density to be employed in that cell.

FIG. 10 shows an alternative method of connecting the cells or cell sections in which the polarity of the current bars on either side of the cells are swapped over (anode bars are swapped with cathode bars) to make for easier and shorter connections, in one embodiment of the invention.

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. A system or an apparatus to which the invention is related may comprise at least one of the embodiments of the invention described hereinbefore.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims. 

1. A system for electrolytic processing of a metal, the system comprising a rectifier and at least two electrolysis cells for the metal, wherein the at least two cells comprise at least three anodes and at least two interleaved cathodes, the system being characterized in that: at least one of the anodes and the cathodes of a first cell have an electrical connection to a terminal of the rectifier via a first electrical path having a first resistance; at least one of the anodes and the cathodes of a second cell have an electrical connection to a terminal of the rectifier, respectively, via a second electrical path having a second resistance; the second resistance is configured to be higher than the first resistance; and the system further comprises a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell.
 2. The system according to claim 1, where the first electrical path and the second electrical path comprise metal conductors.
 3. The system according to claim 2, wherein the second electrical path comprises a resistor and an anode or cathode bar in series, the anode or the cathode bar being connected to each anode or cathode, respectively, of the second cell.
 4. The system according to claim 2, wherein the second electrical path comprises an anode bar to which are connected electrical paths for each of the at least three anodes of the second cell, the electrical paths for each of the at least three anodes having respective resistors.
 5. The system according to claim 2, wherein the second electrical path comprises an anode or a cathode bar, respectively, of the first cell.
 6. The system according to claim 1, wherein the metal is copper.
 7. The system according to claim 1, wherein the first cell and the second cell are liberator cells.
 8. The system according to claim 1, wherein the system further comprises an intermediate voltage supply configured to supply local converters.
 9. The system according to claim 1, wherein the electrolytic process is electrowinning or electrorefining.
 10. A system for electrolytic processing of a metal, the system comprising a rectifier and at least two electrolysis cells for the metal and a rectifier, wherein a first cell comprises a plurality of cathodes interleaved between a plurality of anodes and a second cell comprises a plurality of cathodes interleaved between a plurality of anodes, the system being characterized in that: the anodes of the first cell have an electrical connection to a positive terminal of the rectifier via a first electrical path having a first resistance; the anodes of the second cell have an electrical connection to a positive terminal of the rectifier via a second electrical path having a second resistance; the number of anodes and cathodes in the second cell is configured to be higher than the number of anodes and cathodes in the first cell to diminish a difference between the first resistance and the second resistance; and the system further comprises a channel for electrolyte from the first cell to the second cell, the electrolyte containing the metal in a dissolved ionic form, metal concentration in the first cell being higher than in the second cell.
 11. The system according to claim 10, wherein the metal is copper.
 12. The system according to claim 10, wherein the first cell and the second cell are liberator cells.
 13. The system according to claim 10, wherein the system further comprises an intermediate voltage supply configured to supply local converters.
 14. The system according to claim 10, wherein the electrolytic process is electrowinning or electrorefining . 